The present invention relates to a yeast cell utilizing solely galactose as a carbohydrate source in a presence of glucose.
Conventional yeast cells have activatable enzymes that enable the yeast cells to metabolize a variety of sugars. The sugars include glucose, maltose and galactose. Regulation of activities of the enzymes is based upon a selective activation and suppression of enzyme activity. The activities of enzymes aiding in metabolism of sugars other than glucose are suppressed when the yeast cell is grown in a media that includes glucose as a carbohydrate source for the yeast cell.
The suppression in enzyme activity occurs by one of two mechanisms. A first mechanism, glucose inactivation, rapidly inhibits the function of some enzymes and other proteins by modification and/or degradation of the proteins. A second mechanism, glucose repression, reduces the expression of many genes making enzymes and regulating enzyme activity at a transcriptional level.
Glucose repression in yeast cells is similar to a metabolic process termed "catabolite repression" in the organism, Escherichia coli. The term "catabolite repression" reflects a belief that repression of gene transcription is caused not so much by glucose itself but by glucose catabolites, as described by B. Magasanic in "Catabolite repression," in Cold Spring Harbor Symp, Quant. Biol. 26: 249 (1962). In the present application, the term "glucose repression" is used interchangeably with "catabolite repression".
The expression of a large number of yeast cell genes is subject to glucose repression. However, the degree of glucose repression varies from gene to gene. For example, expression of genes involved in galactose utilization, galactose genes (GAL genes), is repressed at least 1000-fold by glucose. Expression of genes involved in maltose metabolism, maltose genes, is repressed by about 15 fold as described by M. Johnston et al. in The Molecular and Cellular Biology of the Yeast Saccharomyces (1992) at 226.
Glucose appears to have two effects on expression of GAL genes. The first effect is that an addition of glucose to a galactose-based substrate media causes a yeast culture growing on the galactose enriched media to display a nearly complete but transient repression of GAL gene expression. The second effect is that GAL enzyme synthesis subsequently resumes at a reduced rate as described by Adams in an article, "Identification of glucokinase in Saccharomyces cerevisiae: Kinetics of induction and glucose effects," J. Bacteriol III: 308 (1972).
Because of glucose repression, conventional yeast cultures grow on glucose in three phases in a batch culture as described by E. Jones, et al. in The Molecular and Cellular Biology of the Yeast Saccharomyces (1992). In a first or rapid phase of growth, glucose is fermented with a concomitant glucose repression of gene expression. Shortly before the glucose is depleted, the culture enters a second growth phase during which glucose-repressed genes become de-repressed, adapting the culture for subsequent oxidation of an ethanol metabolite that accumulates during glucose utilization. De-repression of the synthesis of some enzymes begins well before glucose exhaustion and reaches maximal levels during growth on ethanol. Growth in a third or last phase is slow and ceases with the exhaustion of the available ethanol.
Because of a great adaptability and versatility of yeast strains generally, predicting performance of a particular yeast strain when the yeast strain is exposed to a complex nutrient substrate has been exceedingly difficult. On one hand, although a yeast strain might have a preference for a carbohydrate such as galactose, the yeast strain preference for galactose is undermined when the yeast is exposed to a substrate having a concentration of glucose. The mere presence of glucose tends to repress galactose metabolism. The yeast strain may be prevented from utilizing any carbohydrate at all. Alternatively, the yeast strain may revert to glucose metabolism.